This invention relates to an image processing method and apparatus. More particularly, the invention relates to color reproduction processing of a color printer.
An image processing method based upon a color dot model is well known as an image processing method using a color printer.
FIG. 20 is a block diagram illustrating the architecture of an image processor in a color printer according to the prior art. The luminance signals of R, G, B entered from an input terminal 101 are each converted to density signals of C, M, Y by a density converter 102. A black density signal K is produced by a black generator 103 from the density signals resulting from the conversion, and a dot area signal subjected to undercolor removal and compensated for crosstalk components of the density signals by a masking/UCR unit 104 is obtained. Linearity between the dot area signal and output density based upon dot gain is corrected by an output gamma corrector 105, each color component is binarized by a binarizing unit 106, and the resulting dot patterns are outputted to a printer engine (not shown) from an output terminal 107.
The density converter 102 and output gamma corrector 105 usually are constituted by a look-up table (hereinafter referred to as a "LUT").
In the example of the prior art described above, binarization processing is executed for each of the C, M, Y, K planes. Consequently, the overlapping of the C, M, Y, K dots is random, it is very difficult to achieve conformity of color reproduction by masking and difficult to limit the number of dots on dots.
Further, since density conversion, black generation, masking/UCR, output gamma correction and binarization processing are executed independently of one another, processing takes time or a large amount of hardware (calculations) is required.
The specifications of U.S. Pat. Nos. 5,070,413 and 5,270,808 describe technology for converting input color image data to output color image data in three-dimensional color space. The disclosed technology obtains 1-bit output data for each of Y, M, C, K from 8-bit input data for each of R, G, B. According to this approach, in order to determine which output color, among 16 output colors represented by one bit for each of Y, M, C, K, the input data represented by eight bits for each of R, G, B in three-dimensional color space is nearest to, 16 distances between the input data and the output colors are obtained and the output color for which the distance is smallest is decided upon as being the output color.
In this case, processing takes time. Accordingly, there is a need for high-speed processing and processing for providing higher image quality.